1. Field of the Invention
The present disclosure relates generally to a voltage reference and, more particularly, to a low power voltage reference.
2. Description of the Related Art
Today, systems, such as battery-powered systems, are usually designed to enter a low-power mode when the systems are not being utilized. When in the low-power mode it is desirable for the systems to consume a relatively small amount of power. In systems that utilize voltage references, it is desirable for the voltage references to be designed to consume a relatively small amount of power during normal operation, as well as when the systems are in a low-power mode. Voltage references are used in a variety of different applications. For example, analog-to-digital converters (ADCs), digital-to-analog converters (DACs), oscillators, flash memories, and voltage regulators usually require a voltage reference that is relatively insensitive to temperature, power supply, and load variations. The resolution of an ADC or a DAC, for example, is generally limited by the precision of an associated reference voltage over a power supply voltage range and operating temperature range.
Traditionally, bandgap voltage references have employed bipolar junction transistors (BJTs) to generate a relatively temperature independent reference voltage. In general, bandgap voltage references exhibit a relatively high power supply rejection ratio (PSRR) and a relatively low temperature coefficient. To reduce power consumption of integrated circuits (ICs), many IC designers have migrated from bipolar to complementary metal-oxide semiconductor (CMOS) processes. While bipolar CMOS (BiCMOS) processes may be used in the design of a bandgap voltage reference, BiCMOS devices are relatively expensive, as compared to CMOS devices. Moreover, bandgap voltage references have usually employed ratiometric related resistors. In a bandgap voltage reference, in order to provide for relatively low current, one resistor of the bandgap voltage reference is typically many times the size of another resistor. It should be appreciated that larger area resistors increase an area of an associated IC which, in turn, has increases the cost of the associated IC.
U.S. Patent Application Publication No. 2006/0001412 (hereinafter “the '412 application”) discloses a voltage reference that is fabricated exclusively using CMOS processes. The voltage reference of the '412 application employs a current generator that provides a proportional to absolute temperature (PTAT) current. A stack of serially coupled metal-oxide semiconductor field-effect transistors (MOSFETs) is coupled between the current generator and a common point, i.e., ground. The stack of MOSFETs have a transimpedance which has a temperature coefficient that is opposite in polarity to a temperature coefficient of an internal resistance of the current generator. As such, the voltage reference of the '412 application provides a reference voltage that is relatively stable over temperature.
Referring to FIG. 1, a conventional voltage reference 100, disclosed in the '412 application, is illustrated. The reference 100 includes a current mirror (including p-channel MOSFETs M1, M2, and M3), n-channel MOSFETs M4 and M5, a resistor R1, and an output load 102, which includes a stack of saturated and linear n-channel MOSFETs that conduct a proportional to absolute temperature (PTAT) current. In a disclosed embodiment, the MOSFET M1 is sized at ‘X’, the MOSFET M2 is sized at ‘2X’ and the MOSFET M3 is sized at ‘X’. As such, the current flowing through the MOSFETs M1 and M3 is I1 and the current flowing through the MOSFET M2 is 2I1. The currents I1 and 2I1 are PTAT currents and a magnitude of the current 2I1 (and correspondingly the current I1) is determined by a difference in gate-to-source voltage (delta−Vgs) of the MOSFETs M4 and M5 divided by a value of the resistor R1. The MOSFET M4 is diode-connected and conducts the current I1 provided via the MOSFET M1. The MOSFET M5 conducts the current 2I1 provided via the MOSFET M2. Similarly, the load 102 conducts the current I1 provided via the MOSFET M3. The resistor R1 is coupled between the MOSFET M5 and a common point (VSS). An impedance of the load 102 varies as a function of temperature to maintain a reference voltage (VREF) at a substantially temperature independent level. That is, when the PTAT current increases, the impedance of the load 102 decreases. Moreover, when the PTAT current decreases, the impedance of the load 102 increases.
The load 102 includes a stack 104, which includes multiple diode-connected n-channel MOSFETs in series. In this configuration, the MOSFETs of the stack 104 operate in a saturated region (i.e., Vgs>Vth and Vds>Vgs−Vth, where Vgs is the gate-to-source voltage, Vth is the threshold voltage, and Vds is the drain-to-source voltage). A drain-to-source voltage (Vds) of each of the MOSFETs of the stack 104 is equal to a gate-to-source voltage (Vgs), due to the manner in which the MOSFETs are connected. Each of the MOSFETs of the stack 104 may be switched out of the circuit by activating an appropriate p-channel MOSFET to modify a level of the reference voltage (VREF).
The load 102 also includes two variable length MOSFET structures 106, which effectively provide a MOSFET with a variable length for a given width. The variable length MOSFET structures 106 are serially connected between the stack 104 and another variable length MOSFET structure 108. Each of the variable length MOSFET structures 106 includes a first diode-connected n-channel MOSFET that is serially coupled to a string of n-channel MOSFETs. Gates of the MOSFETs, of the MOSFET string, are coupled to a gate of the first diode-connected MOSFET. In the disclosed configuration, the MOSFETs of the structures 106 operate in a saturated region such that the gate-to-source voltage (Vgs) of the structures 106 may be varied by varying the number of MOSFETs in the structure 106. That is, selected MOSFETs of the structure 106 may be shorted by activating an appropriate p-channel MOSFET to change a length of the structure 106 to affect a change in the gate-to-source voltage and an associated change in the reference voltage. The structure 108 is similar to the structures 106, with the exception that a first MOSFET in the string is not diode connected. The structures 106 function as a linear drain-to-source resistor (rds) with a positive temperature coefficient (PTC). Moreover, the structure 108 operates in a linear region (i.e., Vgs>Vth and Vds<Vgs−Vth) as the gates of the MOSFETs of the structure 108 are coupled to an output of the stack 104. As such, a voltage across the structure 108 is the drain-to-source voltage (Vds) of the structure 108. To effect a temperature coefficient adjustment for the load 102, selected MOSFETs of the structure 108 may be shorted.
U.S. Pat. No. 6,919,753 (hereinafter “the '753 patent”), discloses a voltage reference that is also fabricated exclusively using CMOS processes. With reference to FIG. 2, a voltage reference 200, configured according to the '753 patent, includes two p-channel metal-oxide semiconductor field-effect transistors (MOSFETs) M1 and M2, two n-channel MOSFETs M3 and M4, and two resistors R1 and R2, which may be variable resistors. The MOSFETs M1 and M2 provide a current mirror. The MOSFETs M3 and M4, whose gates are interconnected, and the resistors R1 and R2 provide a temperature compensation circuit. The temperature compensation circuit is configured to generate a reference voltage (VREF) whose level is relatively independent of temperature. The MOSFET M4, which is a diode-connected MOSFET, receives a proportional to absolute temperature (PTAT) current I1 provided via the MOSFET M2. In the disclosed configuration, the MOSFET M3 and the resistor R1 also conduct a current I1 provided via the MOSFET M1 (i.e., the current mirror including the MOSFETs M1 and M2 is a 1:1 current mirror). The current I1 is a PTAT current and a magnitude of the current I1 is determined by a difference in gate-to-source voltage (delta−Vgs) of the MOSFETs M3 and M4 divided by a value of the resistor R1. In a disclosed configuration, the MOSFETs M3 and M4 operate near a subthreshold region (i.e., gate-to-source voltage (Vgs) is approximately equal to a threshold voltage (Vth)). The gate-to-source voltage (Vgs) of the MOSFET M4 is complementary to absolute temperature (CTAT). The reference 200 provides a reference voltage that includes a first term that is a proportional to absolute temperature (PTAT) voltage and a second term that is a complementary to absolute temperature (CTAT) voltage. The first term corresponds to a voltage drop across the resistor R2 and the second term corresponds to a threshold voltage of the diode-connected MOSFET M4. A reference voltage provided by the reference 200 is determined by the ratios of the MOSFETs M3 and M4 and values selected for the resistors R1 and R2. The reference 200 utilizes an operating current of several microamperes and requires matching of the MOSFETs M3 and M4 and proper selection of values for the resistors R1 and R2 to provide a reference voltage that is relatively stable.
In a typical bandgap voltage reference, a change in base-to-emitter voltage (delta−Vbe) of a transistor provides a PTAT contribution to a reference voltage and a base-to-emitter voltage (Vbe) of a transistor provides a CTAT contribution to the reference voltage that counteracts the PTAT contribution. Similarly, voltage references that employ MOSFETs operating in a weak inversion region (i.e., an effective gate-to-source voltage (Veff, where Veff=Vgs−Vth) between about 3 Ut to 5 Ut, where Ut is the thermal voltage) have been configured to employ a difference in gate-to-source voltage (delta−Vgs) to provide a PTAT contribution to a reference voltage and a gate-to-source voltage (Vgs) to provide a CTAT contribution to the reference voltage. In either case, summing the PTAT contribution and the CTAT contribution using a selected ratio provides a reference voltage that is relatively independent of process and temperature variations. However, weak bipolar junction transistors created in CMOS processes usually require a minimum operating current of about one microampere in order to bias the bipolar junction transistors at a relatively high beta operating point. Furthermore, MOSFETs operating in a ‘weak inversion region’ are highly sensitive to threshold voltage (Vth) variations, as a current conducted by a MOSFET operating in a ‘weak inversion region’ is exponentially proportional to a gate-to-source voltage (Vgs) minus a threshold voltage (Vth) divided by a thermal voltage (Ut), i.e., exp(Vgs−Vth/Ut).
What is needed is a voltage reference that requires a relatively low operating power. It would also be desirable if the voltage reference was designed to be substantially insensitive to threshold voltage variations.
In the following detailed description of exemplary embodiments of the invention, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific exemplary embodiments in which the invention may be practiced.